Distributed sensor

ABSTRACT

A laminar sensor ( 1 ) for detecting changes on a laminar substrate ( 35 ). The sensor includes a laminar sheet ( 3 ) which has a first surface ( 5 ) and a second opposite surface ( 7 ), and is made from a conductive polymer composition which exhibits temperature dependent resistance behavior, preferably PTC behavior. A plurality of sensing elements ( 12 ) are electrically connected, preferably in series, on the sensor. Each sensing element is formed as an electrode pair containing a first electrode and a second electrode. The first and second electrodes ( 9, 11 ) may be on the same surface of the laminar sheet or on opposite surfaces of the sheet. Two electrical leads ( 17, 19 ) are present for connecting the sensing elements into a circuit, which may be used to detect changes in resistance which occur when a sensing element is exposed to an elevated temperature, a change in pressure, or a solvent.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.09/182,590, filed Oct. 28, 1998, now U.S. Pat. No. 6,137,669, and is theNational Stage of International Application No. PCT/US99/25351, thedisclosure of each of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to sensors, including temperature sensors.

2. Introduction to the Invention

A wide variety of electronic components and other articles are subjectto damage if exposed to elevated temperature. It is, therefore, oftenimportant to be able to determine if a component has been subjected tosuch temperature. Various detection techniques, e.g. thermochromicmaterials which change color when exposed to a specific temperature,have been proposed for this purpose. Such techniques suffer from therequirement that the article must be visible in order to detect thecolor change, and thus are ineffective when the article is enclosed.Various electronic detectors, designed to identify an electrical changeresulting from a high temperature, have also been proposed. Suchdetectors may not be able to determine whether a particular part of anarticle has been exposed to a high temperature, relying instead on theaverage over the entire surface. In addition, sensors which are able tomaintain direct contact with the substrate, even when the substrate isnot flat, are desirable. Such sensors would have sufficient flexibilitythat they could provide two-dimensional sensing over a large surface,and be able to be bent over an edge to provide three-dimensionalsensing.

Conductive polymer compositions exhibiting a positive temperaturecoefficient of resistance (PTC) effect are well known. Such compositionscomprise a polymeric component, and dispersed therein, a particulateconductive filler. At low temperatures the composition has a relativelylow resistivity. However, when the composition is exposed to a hightemperature, due for example, to a high current condition, theresistivity of the composition increases, or “switches”, often byseveral orders of magnitude. The temperature at which this transitionfrom low resistivity to high resistivity occurs in a PTC composition isthe switching temperature, T_(S). T_(S) is defined as the temperature atthe intersection point of extensions of the substantially straightportions of a plot of the log of the resistance of an element preparedfrom the composition as a function of temperature which lie on eitherside of the portion of the curve showing a sharp change in slope.Similarly, a composition exhibiting a negative temperature coefficient(NTC) of resistance will have a switching temperature, T_(S), in theregion at which the resistivity goes from a high to a low value.

The use of a sensor comprising a PTC conductive polymer to detect anovertemperature condition is known. For example, Japanese PatentApplication No. 10-95019, filed Apr. 7, 1998 (K. K. Raychem), thedisclosure of which is incorporated herein by reference, discloses aelongate temperature sensor which can be used to detect overheating in abattery. Batteries which overheat are subject to damage, and in additionmay damage the packaging surrounding them and the components in contactwith them. While overheating may be due to external environmentalconditions, for secondary, i.e. rechargeable batteries, such overheatingmay occur as a result of excessive charging. The overheating may resultin damage to the internal components of the battery, the generation ofgas, and, under extreme conditions, explosion of the battery. Forexample, for nickel-metal hydride batteries, it is desirable to keep thetemperature below 100° C. to avoid the evolution of hydrogen. It is,therefore, important to identify batteries which have been subject tooverheating before damage can occur. In Japanese Patent Application No.10-95019, a sensor is attached to a plurality of batteries. An elongatetape composed of a PTC conductive polymer comprising spaced-apartsensing components and connecting components is in contact with theindividual battery cells. The sensing components are electricallyconnected in series so that the resistance of the sensor is the sum ofthe resistances of each individual sensing component. The sensor ispositioned so that a sensing component is in contact with the externalsurface of a battery cell, and preferably each individual battery cellcontacts a different sensing component. When the battery cells are in anormal, low temperature condition, the resistance of the sensor is low.If, however, one battery cell heats to a temperature above T_(S), theresistance of the sensing component in contact with that battery cellincreases, thus increasing the total resistance of the sensor andindicating that at least one battery has been subject to overheating.

BRIEF SUMMARY OF THE INVENTION

The approach taken in Japanese Patent Application No. 10-95019 requiresthat the entire battery cell heat to a temperature sufficient to causethe PTC conductive polymer composition to switch. This means that ifthere is a relatively small hot spot inside the battery cell, which issufficient to cause damage to a small region of the battery but isinsufficient to heat the entire cell, it will not be detected. Manybatteries, such as lithium ion polymer batteries have a layered sheetconstruction in which an anode and a cathode are separated by aseparator, and in addition comprise an electrolyte. In practice, thelayered sheet is rolled into a cylinder and positioned inside a can toform a battery cell. A hot spot in the center of the cylinder, due, forexample, to inhomogeneities in the anode, cathode, or separator, cancause damage to the electrolyte, which is solvent-based. It is,therefore, desirable to have a sensor which can detect not just thetemperature of the entire battery cell, but rather the temperature ofindividual spots within the battery cell.

In another application, a lithium ion polymer battery, used unrolled inits thin, flat configuration, can be positioned behind the screen of alaptop computer to detect temperature changes. For this application, itis necessary to have an array of sensing elements as a point sensorapplied to one part of the screen may not reflect a change elsewhere onthe screen.

Detecting individual spots on a substrate is also important for articlesother than batteries. It is desirable to have a sensor in which thepattern of the sensing elements can be designed for a specificconfiguration, so that individual components, e.g. individual elementson a printed circuit board, can be in contact with the sensor. Such asensor can be used for situations in which the temperature at one spotis not representative of the entire surface, but for which sensing isstill required. Furthermore, it is desirable to have a sensor which canbe used to detect hot spots over two dimensions and over a large area.We have now found that a laminar sensor comprising a laminar sheetcomprising a conductive polymer composition and a plurality of sensingelements has sufficient flexibility to contact substrates of nonuniformor irregular structure, as well as the ability to detect temperaturechanges over a broad area. In addition, the sensor can be used to detectresistance changes resulting from pressure or exposure to solvents.Thus, in a first aspect this invention provides a laminar sensor fordetecting changes, e.g. temperature changes, on a laminar substrate, thesensor having a resistance at 20° C. R_(T) and comprising

(1) a laminar sheet which (a) has a first surface and a second oppositesurface, and (b) comprises a conductive polymer composition which (i)exhibits temperature dependent resistance behavior and (ii) has aswitching temperature T_(S);

(2) a plurality of sensing elements and (a) each of which comprises anelectrode pair, said electrode pair comprising a first electrode and asecond electrode, said electrodes being separated from each other and incontact with the laminar sheet, and (b) which are electrically connectedin a resistive network, at least some of said sensing elements connectedin series; and

(3) two electrical leads for connecting the sensing elements into acircuit.

In a second aspect, the invention provides a lithium ion polymer batterywhich comprises

(A) a laminar battery element surrounded by an insulating material, saidbattery element comprising (1) first and second battery electrodes, (2)an anode, (3) a separator, (4) a cathode, and (5) and electrolyte; and

(B) a laminar temperature sensor of the first aspect of the inventionpositioned in direct contact with the insulating material and coveringat least 75% of one laminar surface of the insulating material.

In a third aspect, the invention provides an electrical circuit whichcomprises

(A) a laminar sensor of the first aspect of the invention; and

(B) detection equipment electrically connected to the electrical leadsto detect a change in the sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated by the drawings in which

FIG. 1 is a top schematic view of a sensor of the invention;

FIG. 2 is a cross-sectional view along line 2—2 of FIG. 1;

FIGS. 3 and 4 are top schematic views of sensors of the invention;

FIG. 5 is an electrical circuit containing a sensor of the invention;

FIGS. 6 and 7 are top schematic views of other sensors of the invention;and

FIG. 8 is a cross-sectional view of another sensor of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The sensor of the invention comprises a laminar sheet comprising aconductive polymer composition which exhibits temperature dependentresistance behavior. The composition preferably exhibits PTC behavior,but in some applications, it is preferred that a composition exhibitingNTC behavior, i.e. a change from high to low resistivity with increasingtemperature, be used. The conductive polymer composition comprises apolymeric component, and dispersed therein, a particulate conductivefiller. The polymeric component comprises one or more polymers, one ofwhich is preferably a crystalline polymer having a crystallinity of atleast 10% as measured in its unfilled state by a differential scanningcalorimeter. Suitable crystalline polymers include polymers of one ormore olefins, particularly polyethylene such as high densitypolyethylene; copolymers of at least one olefin and at least one monomercopolymerisable therewith such as ethylene/acrylic acid, ethylene/ethylacrylate, ethylene/vinyl acetate, and ethylene/butyl acrylatecopolymers; melt-shapeable fluoropolymers such as polyvinylidenefluoride (PVDF) and ethylene/tetrafluoroethylene copolymers (ETFE,including terpolymers); and blends of two or more such polymers. Forsome applications it may be desirable to blend one crystalline polymerwith another polymer, e.g. an elastomer or an amorphous thermoplasticpolymer, in order to achieve specific physical or thermal properties,e.g. flexibility or maximum exposure temperature. The polymericcomponent generally comprises 40 to 90% by volume, preferably 45 to 80%by volume, especially 50 to 75% by volume of the total volume of thecomposition.

The particulate conductive filler which is dispersed in the polymericcomponent may be any suitable material, including carbon black,graphite, metal, metal oxide, conductive coated glass or ceramic beads,particulate conductive polymer, or a combination of these. The fillermay be in the form of powder, beads, flakes, fibers, or any othersuitable shape. The quantity of conductive filler needed is based on therequired resistivity of the composition and the resistivity of theconductive filler itself. For many compositions the conductive fillercomprises 10 to 60% by volume, preferably 20 to 55% by volume,especially 25 to 50% by volume of the total volume of the composition.

The conductive polymer composition may comprise additional components,such as antioxidants, inert fillers, nonconductive fillers, radiationcrosslinking agents (often referred to as prorads or crosslinkingenhancers, e.g. triallyl isocyanurate), stabilizers, dispersing agents,coupling agents, acid scavengers (e.g. CaCO₃), or other components.These components generally comprise at most 20% by volume of the totalcomposition.

The composition used in the laminar sheet preferably exhibits positivetemperature coefficient (PTC) behavior, i.e. it shows a sharp increasein resistivity with temperature over a relatively small temperaturerange. In this application, the term “PTC” is used to mean a compositionwhich has an R₁₄ value of at least 2.5 and/or an R₁₀₀ value of at least10, and it is preferred that the composition should have an R₃₀ value ofat least 6, where R₁₄ is the ratio of the resistivities at the end andthe beginning of a 14° C. range, R₁₀₀ is the ratio of the resistivitiesat the end and the beginning of a 100° C. range, and R₃₀ is the ratio ofthe resistivities at the end and the beginning of a 30° C. range.Generally the compositions used in devices of the invention showincreases in resistivity which are much greater than those minimumvalues. It is preferred that these compositions have a PTC anomaly at atleast one temperature over the range from 20° C. to (T_(n)+5° C.) of atleast 10¹, preferably at least 10², particularly at least 10³,especially at least 10⁴, i.e. the log[resistance at (T_(n)+5°C.)/resistance at 20° C.] is at least 1.0, preferably at least 2.0,particularly at least 3.0, especially at least 4.0, where T_(m) is themelting temperature of the polymeric component as measured at the peakof the endotherm of a differential scanning calorimeter (DSC) trace.(When there is more than one peak, as for example in a mixture ofpolymers, T_(m) is defined as the temperature of the highest temperaturepeak.)

Suitable conductive polymer compositions exhibiting PTC behavior aredisclosed in U.S. Pat. Nos. 4,237,441 (van Konynenburg et al), U.S. Pat.No. 4,545,926 (Fouts et al), U.S. Pat. No. 4,724,417 (Au et al), U.S.Pat. No. 4,774,024 (Deep et al), U.S. Pat. No. 4,935,156 (vanKonynenburg et al), U.S. Pat. No. 5,049,850 (Evans et al), U.S. Pat. No.5,250,228 (Baigrie et al), U.S. Pat. No. 5,378,407 (Chandler et al),U.S. Pat. No. 5,451,919 (Chu et al), U.S. Pat. No. 5,582,770 (Chu etal), U.S. Pat. No. 5,701,285 (Chandler et al), and 5,747,147 (Wartenberget al), and in copending, commonly assigned U.S. application Ser. No.08/798,887 (Toth et al, filed Feb. 10, 1997), now U.S. Pat. No.6,130,597, the counterpart of which is published as International PatentPublication No. WO97/29711, published Sep. 26, 1996. The disclosure ofeach of these patents and applications is incorporated herein byreference.

The laminar sheet has a first surface and a second opposite surface. Thesheet can be of any thickness, but for many applications in which it isdesirable that the sensor be flexible, it is preferred that the sheet berelatively thin, i.e. have a thickness of at most 1.0 mmn (0.040 inch),preferably at most 0.76 mm (0.030 inch), particularly at most 0.51 mm(0.020 inch, e.g. 0.08 to 0.25 mm (0.003 to 0.010 inch). The sheet ispreferably a solid layer, but it may contain slits or openings in orderto accommodate attachment means to the substrate or to enhance theflexibility or fit onto a substrate. The sheet may be crosslinked, e.g.by irradiation or chemical means. The sheet may comprise a singleconductive polymer, or different conductive polymer compositions may beused in different sections of the sheet to provide different thermal orelectrical capabilities.

A plurality of sensing elements is attached to the laminar sheet. Eachsensing element in the preferred embodiment comprises an electrode pairin which a first electrode is attached to the first surface of the sheetand a second electrode is attached to the second surface of the sheet.In this embodiment, a current flow would be through the thickness of thesheet. The electrodes comprise an electrically conductive material, e.g.a metal foil, a conductive ink, or a metal layer which has been appliedby plating or other means. The attachment of the electrodes to thesurface of the sheet may be either direct, e.g. a metal foil orconductive ink in direct physical contact with the sheet, or indirect;e.g. a metal layer applied via an adhesive or tie layer. In anotherembodiment, the first and second electrodes can be positioned on thefirst surface, so that any current flow is parallel to the firstsurface. The first and second electrodes are generally physically (i.e.spatially) separated from one another.

The sensing elements are electrically connected in a resistive network,and at least some of the sensing elements in the resistive network areelectrically connected in series. Preferably a first connectingcomponent connects two first electrodes, while a second connectingcomponent connects two second electrodes. In an embodiment in which thefirst and second electrodes are on opposite surfaces of the sheet, thesecond connecting component is positioned between, and on the oppositesurface of, two first electrodes so that one of the two secondelectrodes to which it is connected overlaps one of the two firstelectrodes. In some embodiments, the sensor may comprise a number, i.e.at least two, groups of sensing elements in an array. In the array, eachof the sensing elements within each group is connected in series, buteach group is not electrically connected to some or all of the othergroups. The group may be in the form of a line or any other pattern, andgroups may be arranged in the array, e.g. in the form of a grid, and maybe connected in parallel. Each group comprises at least two sensingelements, but generally there are more. This design is particularlyuseful when the sensor is intended to have different densities ofsensing elements in different sections, e.g. when particular sections ofa substrate have a greater tendency to overheat than others and greaterprecision is desired in some sections. Furthermore, this design allowsthe sensor to be used in a multiplexing mode. In this process, theresistance of different groups, e.g. lines, of sensing elements isscanned, e.g. line by line in both an x and a y direction, and theresistance values of each scan are compared to a previous scan. Amathematical algorithm can be used to identify a hot spot and itslocation.

The resistive network may also comprise other circuit elements, e.g.components such as capacitors, diodes, switches, and fixed resistors.Such components may be used to “tune” the electrical response to achievecertain conditions such as maximum sensitivity, spatial accuracy,optimal time response, and the lowest power consumption.

For a sensor in which there is only one line or group of sensingelements it is preferred that the total surface area of the firstelectrodes is at least 10%, preferably at least 20% of the total surfacearea of the first surface, and is at most 80%, preferably at most 70%,of the total surface area of the first surface.

Also present are two electrical leads suitable for connecting thesensing elements into a circuit. The circuit may comprise sensingequipment for detecting a resistance change, or it may compriseconventional components, e.g. a power source or load resistance. Theseleads may be in the form of metal pads on the sheet, similar in materialto the electrodes, or they may be wires or other conductive elements.

It is preferred that the first and second electrodes and the first andsecond connecting components be the same material. Particularlypreferred for electrodes and connecting components are electrodepositedmetal foils such as nickel, copper, or nickel-copper foils, which may belaminated to the sheet. A conventional photolithographic process can beused to remove metal from some or all of the regions not intended to beelectrodes, connecting components, or electrical leads. Some metal maybe retained in various sections of the sensor for thermal dissipation oras a reinforcing element. Such metal is not electrically connected tothe sensing elements. In an alternative process, the electrodes andconnecting components can be applied by screen-printing.

The shape of individual sensing elements may be the same or different onthe sensor, although it is preferred that the shape of the first andsecond electrodes in an individual sensing element be the same.Depending on their shapes, the sensing elements may have the same ordifferent resistance, R_(S), at 20° C. It is important that the sensingelements be sufficiently low in resistance (e.g. sufficiently large) sothat the total series resistance at 20° C. of the circuit R_(T) is lowenough that if one sensing element trips and goes into the highresistance state, the total resistance of the sensor will reflect thischange with sufficient resolution. The amount of resistance changerequired to indicate an overheating condition, pressure change, orexposure to solvents is a function of the type of sensing equipmentused. In the preferred embodiment in which the majority of the sensingelements or all of the sensing elements are in series, it is preferredthat when at least one sensing element is exposed to a temperaturegreater than T_(S), the resistance of the sensor is at least 1.1R_(T),preferably at least 1.3R_(T), particularly at least 1.5R_(T). Becausethe sensitivity of the sensor is a function of the number and resistanceof the sensing elements, and because larger increases in resistance willmean that the total change in resistance of the sensor when a sensingelement trips is larger, compositions with higher PTC anomaliesgenerally are preferred. For example, in a sensor with 100 sensingelements, each with a resistance of 1 ohm, R_(T) will be 100 ohms. Ifone sensing element increases in resistance by one decade, i.e. to 10ohms, the sensor resistance will be 109 ohms, i.e. 1.09R_(T). If the onesensing element increases in resistance by two decades, i.e. to 100ohms, the sensor resistance will be 199 ohms, i.e. 1.99R_(T). A threedecade resistance change in one sensing element, to 1000 ohms, will givea sensor resistance of 1099 ohms, i.e. 11R_(T).

Sensors of the invention can be used to detect temperature changes onany type of substrate, but are particularly useful for detecting changeson a laminar substrate, such as a battery, a hot plate, a heating pad,an electric motor case, or a printed circuit board. Due to its laminarstructure and its flexibility, the sensor can be in direct physicalcontact with the substrate. Although the sensor may cover only a part ofthe substrate, it is particularly useful when the sensor covers asubstantial part of the substrate, i.e. at least 50%, preferably atleast 60%, particularly at least 75% of one surface of the substrate.

Although the sensor of the invention is primarily intended to serve as apassive component on a substrate, under certain circumstances, if theresistance of the sensor is sufficiently low, it may be possible to passcurrent through the sensor and use it both to detect temperature changesand to act as an overcurrent protection device. For this application,the sensor is connected in series in a circuit with a power source andother electrical components which provide a load resistance, and sensingequipment is connected to the sensor in a separate sensing circuit. Forthis application, it is preferred that the sensing elements berelatively large in size so that the sensor resistance is low. Theactual size of the sensing elements will be a function of the maximumcircuit resistance which is often dictated by the maximum voltage dropin the system, and the required hold current, i.e. the maximum amount ofcurrent the device can pass without tripping, for the circuit.

It is generally preferred that the resistance of the sensor be as low aspossible. This is particularly desirable when the sensor is actuallypart of a circuit and is simultaneously acting as an overcurrentprotection device and a temperature sensing device. Under thesecircumstances, it is important that the resistance of the sensor be lowwith respect to the circuit to be measured. Thus the resistivity at 20°C. of the composition in the laminar sheet is preferably low, i.e. lessthan 100 ohm-cm, preferably less than 20 ohm-cm, particularly less than10 ohm-cm, especially less than 5 ohm-cm.

The sensor of the invention is particularly useful when the substrate isa laminar battery element, in particular a lithium ion polymer batteryelement. The sensor is sufficiently flexible that it can be rolled intoa cylinder, as is commonly done with a lithium ion polymer batteryelement, and then can detect temperature or other changes which occur atvarious spots on the battery element and which result in a change inresistance.

Although the sensor of the invention is suitable for use in detecting achange in resistance, it also can be used to detect a rate of change ofresistance.

In other applications, sensors of the invention can be used to detectchanges in pressure, as the resistance of the sensing elements willchange with pressure. Sensors of the invention can also be used todetect the presence of solvents if the conductive polymer composition isselected to swell (and thus change resistance) when exposed to asolvent.

The invention is illustrated by the drawings in which FIG. 1 is a topschematic view of a sensor of the invention and FIG. 2 is across-sectional view along line 2—2 of FIG. 1. Sensor 1 has a laminarsheet 3 composed of a conductive polymer composition and having firstand second surfaces 5,7. Attached to first surface 5 are firstelectrodes 9. Two first electrodes 9 are electrically connected byconnecting component 13. Attached to second surface 7 are secondelectrodes 11. Two second electrodes 11 are electrically connected byconnecting component 15. Sensing element 12 is composed of an electrodepair of one first electrode 9 and the one second electrode 11 which isopposite it. First and second electrical leads 17,19 are positioned onfirst surface 5 and provide means for connection to detectionelectronics or a power source. Because all of the sensing elements areconnected in series, only one pair of electrical leads 17,19 is requiredto measure the resistance of the entire sensor.

FIGS. 3 and 4 are top schematic views of the sensors described below inExamples 1 and 2, respectively.

FIG. 5 shows an electrical circuit used in Example 2. In this circuitsensor 1 is electrically in series with power source 21 and loadresistor 23. Detection electronics 25, capable of detecting changes inresistance of the sensor, are connected to sensor 1 in a separatesensing circuit.

FIG. 6 shows a top schematic view of another sensor of the invention, inwhich multiple sensor lines 29 are present. Each line has first andsecond electrical leads 17,19, so that individual measurements of thatline can be made and some determination of the location of the hot spotor site of the detected change can be made.

FIG. 7 shows a top schematic view of another sensor of the invention, inwhich multiple sensor lines are present and the sensing elements arematrixed to provide increased accuracy for determination of the hot spotor site of the detected change. Third and fourth electrical leads 31,33are present to allow additional resistance measurements to be made, e.g.for a multiplexing application.

FIG. 8 shows in cross-section a sensor 1 of the invention positioned onsubstrate 35, e.g. a battery. First and second electrodes 9,11 areattached to the sane surface, first surface 5, of laminar sheet 3.Second electrode 11 is electrically connected in series to another firstelectrode 9 by means of connecting component 13. Sensing element 12 iscomposed of first electrode 9 and its adjacent second electrode 11,separated by the conductive polymer in laminar sheet 3.

The invention is illustrated by the following examples.

EXAMPLE 1

A conductive polymer composition comprising a mixture of 40% by weightethylene/n-butyl acrylate co polymer (Enathene™ 705-009, having amelting temperature of about 105° C. and a density of about 0.922 g/cc,available from Quantum Chemical), 10% by weight high densitypolyethylene (Petrothene™ LB832, having a melting temperature of about135° C. and a density of about 0.954 g/cc, available from QuantumChemical), and 50% by weight carbon black (Raven 430, having a densityof 1.8, available from Columbian Chemicals) was mixed, formed into asheet having a thickness of 0.13 mm (0.005 inch), and laminated oneither side with electrodeposited nickel-copper foil having a thicknessof 0.043 mm (0.0013 inch). The laminate was irradiated to 10 Mrads, andwas then subjected to a photolithographic and etching process similar tothe type described in U.S. Pat. No. 5,864,281 (Zhang et al), thedisclosure of which is incorporated herein by reference. The laminatewas cleaned and photo resists were used to produce masks over the metalfoils in the regions which were to be the sensing elements and theelectrical connection (i.e. connecting components) regions. Theremaining regions of the foils were left exposed and were etched toremove the metal foils in those areas. The masks were then removed. Theetched laminate was cut into pieces 51×76 mm (2×3 inches), each havingthe configuration shown in FIG. 3. For this sensor, fifty-two sensingelements, each about 4 mm (0.18 inch) square and having a resistance ofabout 0.1 ohm, were evenly distributed and electrically connected inseries. The sensing elements covered approximately 28% of each laminarsurface of the sensor. Metallized regions at the edge of the elementwere suitable for use as electrical leads. The resistivity of thecomposition was about 0.5-1.0 ohm-cm; the switching temperature T_(S) ofthe composition as defined above, was about 93° C.

The resistance of the sensor at 20° C. was 4.1 ohms. A heat gun was usedas an external heat source and was applied to various numbers of sensingelements. The temperature of the sensor was monitored using a thermalimaging camera, and the maximum temperature, as well as the resistanceof the sensor, were recorded. The results are shown in Table I.

TABLE I Number of Elements Heated Maximum Temperature (° C.) Resistance(ohms) 0 20 4.1 1 70 8.0 2 100 8.8 ˜20 85 20 ˜40 100 111

EXAMPLE 2

Using the procedure and compositions of Example 1, a sensor having theconfiguration shown in FIG. 4 was prepared. The sensor had dimensions of51×76 mm (2×3 inches), with six sensing elements, each 10×30 mm (0.4×1.2inches), connected in series. The sensing elements covered approximately48% of each laminar surface of the sensor. The resistance of the sensorat 20° C. was 0.042 ohm, each sensor having a resistance of about 0.007ohm.

The sensor was connected in a circuit (as shown in FIG. 5) in serieswith a power supply and a load resistor which limited the currentpassing through the sensor to 5A when a voltage of 18 volts was applied.Under these conditions, the highest temperature detected on the sensorby a thermal imaging camera was 33° C. and the sensor did not trip. Aheat gun was used to apply heat to one sensing element, causing thesensing element to increase in resistance and in temperature to at least95° C. In addition, the thermal derating of the sensor, affected by theincrease in temperature of the sensing element, prevented the sensorfrom continuing to be able to pass 5A and the sensor tripped. The heatgun was removed from the sensing element, leaving the sensor in thetripped state and the sensing element at 95° C. When the power wasremoved from the sensor, the sensor cooled down and reset.

What is claimed is:
 1. A sensor assembly comprising (A) a laminar sensorfor detecting changes on a laminar substrate, the sensor having aresistance at 20° C. R_(T) and comprising (1) a laminar sheet which (a)has a first surface and a second opposite surface, and (b) comprises aconductive polymer composition which (i) exhibits temperature dependentresistance behavior and (ii) has a switching temperature T_(S), (2) aplurality of sensing elements (a) each of which comprises an electrodepair, said electrode pair comprising a first electrode and a secondelectrode, said electrodes being separated from each other andpositioned in contact with and attached to a surface of the laminarsheet, and (b) which are electrically connected in a resistive network,at least some of said sensing elements connected in series, and (3) twoelectrical leads for connecting the sensing elements into a circuit; and(B) a laminar substrate which is a laminar battery element, the sensorbeing positioned directly in contact with and covering at least 75% ofone surface of the substrate.
 2. A sensor assembly according to claim 1wherein the substrate is a lithium ion polymer battery element.
 3. Asensor assembly according to claim 1 wherein the first electrode ispositioned on and attached to the first surface and the second electrodeis positioned on and attached to the second surface.
 4. A sensorassembly according to claim 1 wherein the first and second electrodesare both positioned on and attached to the first surface.
 5. A sensorassembly according to claim 1 wherein the conductive polymer exhibitsPTC behavior.
 6. A sensor assembly according to claim 1 wherein thelaminar sheet has a thickness of at most 1.0 mm (0.040 inch).
 7. Asensor assembly according to claim 1 wherein the conductive polymercomposition has a resistivity of at most 10 ohm-cm.
 8. A sensor assemblyaccording to claim 1 wherein all of the sensing elements are connectedin series.
 9. A sensor assembly according to claim 1 wherein when atleast one sensing element is exposed to a temperature greater thanT_(S), the resistance of the sensor is at least 1.1R_(T).
 10. A sensorassembly according to claim 9 wherein when at least one sensing elementis exposed to a temperature greater than T_(S), the resistance of thesensor is at least 1.3R_(T).
 11. A sensor assembly according to claim 1which detects temperature changes.
 12. A sensor assembly according toclaim 1 wherein the total surface area of the first electrodes is atleast 10% of the total surface area of the first surface and at most 70%of the total surface area of the first surface.
 13. A sensor assemblyaccording to claim 1 which comprises an array comprising at least twogroups of sensing elements.
 14. A sensor assembly according to claim 13wherein the groups comprise lines of sensing elements.